514  
FXUS10 KWNH 191834  
PMDHMD  
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
133 PM EST TUE DEC 19 2017  
 
VALID DEC 19/1200 UTC THRU DEC 23/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION INCLUDING PREFERENCES AND FORECAST CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...CLOSED LOW CURRENTLY EMERGING INTO SOUTHERN PLAINS TODAY  
EVENTUALLY WEAKENING THROUGH THE TN VALLEY WED AND EAST COAST  
THURS...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
THE 12Z NAM REMAINS A BIT STRONGER/COMPACT WITH THE REMAINING  
INNER CORE OF THE WAVE AS IT TRANSLATES ACROSS THE TN VALLEY  
HOWEVER HAS SHIFTED FROM BEING A SLOW MEMBER OF THE GUIDANCE SUITE  
TO PACE ON THE LEADING EDGE OF THE CLUSTER WITH THIS RUN. OVERALL  
THE TIMING IS SMALL ENOUGH WITHIN THE ENSEMBLE SUITE TO KEEP IT  
WITHIN THE PREFERENCE THOUGH PERHAPS WITH A SLIGHTLY LOWER  
WEIGHTING. THE 12Z GFS SHOWED LITTLE VARIATION TO ACCEPT IT ALONG  
WITH THE 00Z ECMWF AS CORE MEMBERS/HIGHER WEIGHTING IN A GENERAL  
MODEL BLEND. CONFIDENCE REMAINS SLIGHTLY ABOVE AVERAGE GIVEN THE  
MODEST DIFFERENCES IN DEPTH/TIMING REMAINING IN THE SUITE.  
 
19Z UPDATE: THE TREND NOTED BY THE NAM/GFS OF SLIGHTLY FASTER  
EASTWARD TRANSLATION WAS NOTED WITH THE ECMWF/UKMET/CMC AS WELL;  
THOUGH THE CMC IS FLATTENING AT BIT QUICKER AND IS A BIT SOUTH OF  
THE OTHERS. THIS REMAINS MINOR TO NOT CHANGE INITIAL  
PREFERENCE/CONFIDENCE.  
 
...BROAD/LOW AMPLITUDE MID-UPPER LEVEL TROUGH CROSSING THE GREAT  
LAKES/NORTHEAST TODAY /WED...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
OVERALL MODEL AGREEMENT CONTINUES TO TIGHTEN WITH ONLY VERY SMALL  
INNER CORE DIFFERENCES NOTED (NAM DEEPER/CMC SLOWER). THESE  
DIFFERENCES DO NOT APPEAR TO HAVE MUCH IMPACT ON THE SENSIBLE  
WEATHER/MASS FIELDS SUGGESTING AN ABOVE AVERAGE CONFIDENCE IN A  
GENERAL MODEL BLEND.  
 
...UPPER TROUGH AXIS CROSSING THE PACIFIC NORTHWEST THIS EVENING  
REACHING THE FOUR CORNERS ON THU...AND ELONGATING FROM WESTERN  
GREAT LAKES TO NM/OLD MEXICO BY FRI WITH...SURFACE CYCLONE/TROF  
CROSSING THE CENTRAL PLAINS ON THU...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS/GEFS/UKMET BLEND (*00* ECMWF)  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
ENSEMBLE AND CURRENT DETERMINISTIC GUIDANCE IS QUITE AGREEABLE  
WITH THE EVOLUTION OF THE DEEP LOW COMING ASHORE TODAY AND EVEN  
THROUGH THE EVOLUTION OF THE ACCELERATING NORTHERN STREAM/SEVERING  
THE SOUTHERN STREAM CLOSED LOW ACROSS THE FOUR CORNERS THROUGH  
EARLY THURSDAY; THOUGH THE 12Z NAM/GFS ARE A BIT QUICKER IN THE  
NORTHERN STREAM WHICH IS IN LINE WITH TYPICAL BIAS. IT IS AFTER  
ABOUT 12/18Z ON THURSDAY AS THE CLOSED LOW BEGINS TO SHEAR/UNFOLD  
INTO A BROAD POSITIVE TILT TROF SOUTHEAST OF THE FOUR CORNER THAT  
MODEL SPREAD BECOMES MORE EVIDENT. THE 12Z NAM HAVING THE FASTER  
NORTHERN STREAM ALSO DRAGS THE SOUTHERN STREAM A BIT FASTER THAN  
PRIOR CYCLES AND IS CLEARLY ON THE LEADING EDGE OF THE ENSEMBLE  
SUITE. THE 00Z CMC IS ON THE OTHER SIDE OF THE SPECTRUM WITH A  
TYPICAL SLOW TO UNFOLD/COMPACT WAVE THROUGH NM BY FRI. THE 00Z  
ECMWF/ECENS MEAN/UKMET ALL GENERALLY FAVOR THIS SLOWER STRONGER  
SOUTHERN STREAM COMPARED TO EARLIER RUNS OF THE GFS... HOWEVER THE  
12Z GFS RUN HAS SLOWED SIGNIFICANTLY TO DEPART FROM THE NAM AND  
MATCH THE ECMWF/UKMET IN TIMING AS WELL AS GENERAL BREADTH OF THE  
WAVE. THIS PROVIDES INCREASED CONFIDENCE IN A 12Z GFS AND 00Z  
ECMWF/UKMET BLEND...UP TO SLIGHTLY ABOVE AVERAGE.  
 
19Z UPDATE: THE 12Z ECMWF TRENDED BACK TOWARD THE 00Z ECENS MEAN  
BUT WAS ALSO SUPPORTED IN TIMING BY THE 12Z GEFS MEAN. STILL ITS  
OVERALL EVOLUTION SEEMS LESS ALIGNED WITH OTHER GUIDANCE...AS IT  
STALLS/PIVOTS SWINGING ITS BASE SHARPLY THROUGH THE SOUTHWEST/NW  
MEXICO FRIDAY...DELAYING THE EASTWARD PROGRESSION AND SUPPRESSING  
THE SURFACE WAVE TOWARD THE BIG BEND. THIS IS OPPOSITE OF THE  
GEFS/ECENS MEAN THAT WHILE SLOWER...ELONGATE/SHEAR THROUGH THE  
LENGTH OF THE TROF AND SUPPORT THE SURFACE WAVE LIFTING UP THE  
SURFACE TROF INTO EASTERN TX BY 00Z SAT. THE 00Z CMC SLOWED EVEN  
FURTHER LEADING TO A CLEAR MORE SYMMETRIC CLOSED LOW...OUT OF  
PHASE WITH THE OTHER GUIDANCE. THE UKMET WHICH HAS BEEN  
FAST...ALSO SLOWED BUT STILL APPEARS MORE GFS-LIKE. AS SUCH WILL  
HEDGE SUPPORTING A 12Z GFS/GEFS AND UKMET BLEND AND INCLUDE *00Z*  
ECMWF IF THERE IS DESIRE TO HAVE AN ECMWF REPRESENTATIVE. WITH  
LOSS OF ECMWF AND INCREASE IN SPREAD CONFIDENCE ONCE AGAIN REDUCES  
TO AVERAGE IN THE BLEND COMING TO FRUITION.  
 
...UPPER TROUGH AXIS MOVING INTO THE PACIFIC NORTHWEST FROM THE  
NORTH ON THU AND STRETCHING ACROSS NORTHERN ROCKIES/NORTHERN  
PLAINS BY FRI...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS/ECMWF BLEND  
CONFIDENCE: AVERAGE  
 
BY THURSDAY... A BROAD POSITIVE TILT TROF REACHES SE AK/SW YUKON  
TERR. BREAKING DOWN THE LARGER SCALE RIDGE. THE TRAILING EDGE AND  
ASSOCIATED JET STREAK DROPS DUE SOUTH ALONG THE COASTAL RANGE IN  
BC REACHING THE US TOWARD EARLY FRI. THE LEADING EDGE OF THE  
HEIGHT FALLS WILL CARVE OUT/BROADEN THE ARCTIC TROF ACROSS  
SASKATCHEWAN AND DESCEND TOWARD THE NORTHERN US PLAINS BY 00Z SAT.  
THE 00Z UKMET IS MOST DISAGREEABLE WITH THE REMAINING GUIDANCE  
GIVEN THE TROF IS MUCH LESS PHASED IN TIMING/ORIENTATION ACROSS SW  
CANADA...WITH THE LEAD PORTION OF THE TROF OUTPACING AND BECOMING  
MORE COMPACT AND NEARLY SEPARATING WITH THE TRAILING PORTION OF  
THE TROF WHICH AS IS EXPECTED MUCH SLOWER THAN OTHER GUIDANCE.  
THE 00Z CMC WHILE MATCHING THE ECMWF QUITE WELL HAS SOME  
INTERACTION ISSUES WITH THE OVERDEVELOPED DOWNSTREAM CLOSED WAVE  
TOWARD THE END OF FRI...WHILE IT MAY BE USEFUL UP UNTIL MIDDAY  
FRIDAY...THE ECMWF/ECENS MEAN ARE GOOD REPRESENTATIVES IN THE  
REMAINING GUIDANCE SUITE TO ACCOUNT FOR IT IN ANY BLEND. THE 12Z  
NAM IS ALSO DECENTLY TIMED INITIALLY BUT SHOWS A STRONGER  
NORTHERLY JET OVER THE NW AND DRIVES THE HEIGHT FALLS WELL SOUTH  
OF THE GFS/ECMWF BY 00Z SAT. ALL CONSIDERED A 12Z GFS/00Z ECMWF  
BLEND BEST REPRESENT THE ENSEMBLES WHILE PRESENTING A MORE  
METEROLOGICALLY SOUND SOLUTION (IN TIMING/ORIENTATION WITH OTHER  
FEATURES). CONFIDENCE IS AVERAGE IN THIS BLEND.  
 
19Z UPDATE: THE UKMET CONTINUES TO BE LEAST PHASED THROUGH THE  
LENGTH OF THE LARGER SCALE TROF. THE 12Z ECMWF TRENDED A BIT MORE  
AMPLIFIED ENTERING THE PACIFIC NW...BUT THIS MAY BE MORE RELATED  
TO THE STALLING DOWNSTREAM...AND IS MINOR OVERALL WHEN COMPARED TO  
THE INITIAL PREFERENCE/12Z GFS. AS SUCH WILL CONTINUE A 12Z  
GFS/ECMWF BLEND AT AVERAGE CONFIDENCE.  
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML  
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
GALLINA  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page